In an engine operating on a four-stroke cycle (FIG. 1), a mixture of fuel and air, trapped above a moving piston in a closed cylinder, is, during the second and third strokes (FIG. 2), subjected to changes in temperature, volume and pressure, whereby the chemical energy of the fuel is partly converted into mechanical energy of the output shaft.
Thermodynamic analysis of the process shows that for maximum conversion efficiency, the combustion of the mixture must take place in the smallest possible volume with the minimum surface area and at the highest possible temperature. This means that the mixture must be compressed prior to the ignition.
In a practical engine the compression is limited by the onset of detonation, a too rapid combustion which can lead to internal damage of the engine, whereas too high a combustion temperature will cause the percentage of nitrous oxides (NO.sub.x) in the exhaust gases to exceed governmentally established limits on exhaust emissions. Within these constraints, the volume and shape of the combustion chamber are selected to provide optimal combustion conditions for the maximum cylinder charge.
During the first stroke (intake stroke, FIG. 2), the downward moving piston creates a lower-than-atmospheric pressure which causes the fuel/air mixture to flow into the cylinder. This gas flow possesses kinetic energy and, as a result, it continues for a short time after the piston reaches BDC. The velocity of the gas flow, as well as the pressure difference that drives it, vary approximately as the square of the engine speed, and the conditions which result in the maximum charge entering the cylinder can, therefore, only prevail at one particular engine speed.
When the engine is developing its maximum torque at speeds higher or lower than that which results in the maximum charge, the cylinder charge is less than maximum, and the volume of the combustion chamber is "too large" for optimal combustion conditions to occur. This discrepancy becomes even more pronounced when the engine operates at part load.
The power output of a spark-ignited (SI) engine can be controlled by varying either the density (throttle valve in inlet duct) or the volume (late or early closing of the intake valve) of the mixture at the beginning of the compression stroke (LIVC, EIVC).
Obviously, compressing less than the maximum mass in the fixed-volume combustion chamber will generate pressures and temperatures that are lower than those reached under maximum output conditions, and the resulting drop in energy conversion efficiency is the main reason for the low thermal efficiency of spark-ignited engines at part load. In order to deliver power with greater efficiency throughout the output range, an engine, therefore, must incorporate a means for varying the volume of the combustion chamber (commonly described as a variable compression ratio or VCR) in proportion to the load and, to a lesser extent, to the speed.
The compression ratio (C/R), which is defined as the ratio between the volumes above the piston at BDC and TDC (see FIG. 1), is no indication of the efficiency of the combustion process, since the condition of the mixture, just prior to the ignition, depends on the closing time of the intake valve, the engine speed and the initial temperature and density at the beginning of the compression stroke.
The effect of VCR on the part-load operation of an IC engine can best be illustrated by a numerical example. FIGS. 3A, 3B and 3C of the drawings show the PV diagrams of an ideal Otto cycle engine under different operating conditions:
FIG. 3A--Knock-limited, max. load
FIG. 3B--Conventional, part load, standard C/R
FIG. 3C--Knock-limited, part load, increased C/R
In the example, the derived values of volume and pressure are based on the following assumptions:
______________________________________ Exponent of (polytropic) compression n = 1.3 and expansion lines, Compression ratio, standard engine C/R = 6 Knock-limited combustion pressure P = 600 psia Ratio of pressure multiplication a = 4 after combustion ______________________________________
In FIG. 3A: EQU P.sub.2 =P.sub.3 /4=600/4=150 psia EQU P.sub.1 =P.sub.2 (V.sub.C /V.sub.T).sup.1.3 =150(1/6).sup.1.3 =14.60 psia EQU P.sub.4 =4P.sub.1 =4(14.60)=58.42 psia
For part load operation, as shown in FIG. 3B, the condition is chosen whereby the cylinder pressure during the compression stroke reaches 14.60 psia when the volume above the piston is (V.sub.C +V.sub.D)/2=V.sub.T /2. Under these conditions, the mass of the mixture trapped in the engine cylinder is 50% of the maximum charge. EQU P.sub.5 =14.60 psia EQU P.sub.6 =(V.sub.T /2V.sub.C).sup.1.3 P.sub.5 =3.sup.1.3 (14.60)=60.92 psia EQU P.sub.7 =4P.sub.6 =4(60.92)=243.68 psia EQU P.sub.8 =P.sub.7 (V.sub.C /V.sub.T).sup.1.3 =243.68(1/6).sup.1.3 =23.73 psia EQU P.sub.9 =P.sub.8 /4=23.73/4=5.93 psia
In FIG. 3C, while keeping the volume of the mixture V.sub.T /2 the same as in FIG. 3B, and P.sub.11 =P.sub.5 =14.60 psia, the compression space is reduced to V.sub.X in order to attain the knock-limited end-compression pressure P.sub.12 =150 psia. EQU P.sub.12 /P.sub.11 =P.sub.2 /P.sub.1 =150/14.60 EQU (V.sub.T /2V.sub.X).sup.1.3 =(V.sub.T /V.sub.C).sup.1.3 =150/14.60 EQU V.sub.X =V.sub.C /2 EQU C/R =V.sub.T /V.sub.C =(V.sub.D +V.sub.C)/V.sub.C =V.sub.D /V.sub.C +V.sub.C /V.sub.C =6 EQU V.sub.D /V.sub.C =6-1=5 EQU C/R'=V.sub.T '/V.sub.X =(V.sub.D V.sub.X)/V.sub.X =2V.sub.D /V.sub.C +V.sub.X /V.sub.X =11 EQU P.sub.13 =4P.sub.12 =4(150)=600 psia EQU P.sub.14 =(V.sub.X /V.sub.T ').sup.1.3 (600)=(1/11).sup.1.3 (600)=26.57 psia EQU P.sub.15 =P.sub.14 /4=26.57/4=6.64 psia
The ideal cycle diagrams presented in FIGS. 3A, 3B and 3C provide a crude approximation to a real engine operating cycle, but since the same simplifications are used in all cases, a comparison of the area enclosed by each diagram (which is proportional to the work done by the gases on the piston) can give an indication of the beneficial effect of VCR under part-load conditions. The area of the diagram represents work produced when the engine is surrounded by a vacuum; in FIGS. 3B and 3C the compression lines drop below the "atmospheric" line of 14.60 psia and the areas enclosed by points P.sub.5, P.sub.9, P.sub.17, and P.sub.11, P.sub.15, P.sub.16, thus represent negative work that results when the motion of the piston is in the opposite direction of the gas pressure acting upon it. Therefore, to determine the area of the diagram representing the net engine output, twice the area enclosed by the negative loop must be subtracted from the calculated values (see FIG. 3D).
In FIG. 3A: ##EQU1##
In FIG. 3B: ##EQU2##
In FIG. 3C ##EQU3##
Assigning 100% to the value of the work performed by an engine at maximum (=A.sub.1), and taking into consideration that the mass of mixture converted at part-load is 50% of the maximum charge, the relative conversion efficiency is: EQU Part-load, standard, 2A.sub.2 /A.sub.1 =2(220.40/624).times.100%=70.6% EQU Part-load, w/VCR, 2A.sub.3 /A.sub.1 =2(360.33)/624.times.100%=115.5%
These results confirm the known fact that in a conventional Otto-cycle engine, at part load, the thermal efficiency is less than at maximum load and show not only the improvement in efficiency resulting from VCR but also that with VCR, the part-load efficiency is higher than the full-load efficiency in a conventional engine.
The explanation lies in the increase of the expansion and compression ratios, following the incorporation of VCR. The relationship between efficiency and C/R is expressed by the formula, e=1-1/C/R).sup.k-1, derived from the air-standard cycle, a simplified simulation of an engine, used in thermodynamic analysis. Although the working medium is air only, which is subjected to adiabatic instead of polytropic processes, the results have proven to be reliable indicators of the relative effect of principal variables, such as C/R. The symbol k which for air has the value 1.4, represents the ratio of the specific heat at constant pressure c.sub.p to the specific heat at constant volume c.sub.v.
Known mechanisms for varying the compression ratio
The formula C/R=(V.sub.C +V.sub.D)/V.sub.C shows that the compression ratio can be varied by changing V.sub.C or V.sub.D, or both. Since varying the cylinder bore is not practical, all designs which vary the cylinder displacement V.sub.D involve some way of varying the engine stroke. No variable displacement engine has been commercially successful, however, and since the proposed invention involves varying the clearance volume V.sub.C only, a discussion of known mechanisms will be limited to this type of construction, which can be divided into two groups: (a) adjustable cylinder head or part hereof; and (b) adjustable piston crown.
The Cooperative Fuel Research (CFR) single-cylinder engine, built by Waukesha, is of the adjustable cylinder head type and is widely used in laboratories to determine the octane and cetane numbers of fuels. The cylinder head, complete with valves and cylinder wall, is adjustable by means of hand-cranked screwjacks even while the engine is running. In order to cope with the piston-induced side loads, the telescoping upper part of the engine must be guided accurately and virtually without backlash, which leads to a heavy and expensive construction. Strictly a research tool, this design is not practical for multi-cylinder engines.
As a starting aid for compression-ignited (CI) or diesel engines, the two-piece combustion chamber, of which a section can be closed off to temporarily increase the C/R, has been in use for many years. Examples of such arrangements are found in SAE Paper No. 870610, W. H. Adams et al., Luria U.S. Pat. No. 4,033,304 and Luria U.S. Pat. No. 4,084,557. In a newer development, the combustion space is equipped with a cylindrical extension carrying a piston which is adjustable from the outside.
A number of problems are associated with this construction:
(a) the C/R is adjustable over only over a small range;
(b) the combustion chamber has an unfavorable volume/surface ratio, which causes higher heat losses and thus a drop in thermal efficiency;
(c) when the movable piston must be cooled to prevent the creation of a hot spot in the wall of the combustion chamber, reliable sealing in the available space becomes difficult.
A two-piece piston developed by the British International Combustion Engine Research Association (BICERA), consists of an inner core, attached to the connecting rod in the usual manner, and an outer shell which is forced upward by engine oil under pressure and inertia forces when the piston approaches TDC, thus reducing the clearance volume V.sub.C. Built-in flow restrictors control the rate of collapse when the high-pressure gases act on the top of the shell at the beginning of the work-stroke, thereby limiting the maximum combustion pressure over a wide range of operating conditions. These pistons have been successfully tried in medium size diesel engines, but in spark-ignited (SI) passenger car engines, the higher cost would be a problem. The increased inertia loads resulting from their weights could require a major bearing redesign.
The piston action is fully automatic and fast but responds to combustion pressures only. Additional factors, their inputs coordinated by a computer, could be used to optimize the C/R of an SI engine. However, the present construction does not enable these refinements.
Engines with either eccentric piston pins or telescoping connecting rods have been proposed. The main problem with these is that they require a highly-loaded mechanism for which very limited space, inside the piston or within the diameter of the connecting rod, is available. It is possible that a satisfactory construction will be found, suitable for very large units (24" bore minimum), but effects of scale seem to preclude a solution in the case of passenger car engines with pistons typically less than 4" piston diameter.
By mounting the main bearings in eccentric housings, the complete assembly of piston, connecting rod and crankshaft can be moved with respect to the cylinder head. Although space constraints in this case are less severe than those in engines of the type described in the preceding paragraph, maintaining perfect bearing alignment while making V.sub.C adjustments requires an extremely rigid, backlash-free design which is difficult to achieve, especially in multi-cylinder engines. Moreover, the connection between the crankshaft and the engine output shaft, as well as auxiliary drives requires Oldham couplings, U-joints or other means to absorb the displacement of the crankshaft centerline.